The potential use of an addressable mask for direct pattern transfer by charged particle beam projection has been investigated since the 1980's. Such masks will be advantageous in the field of particle-beam lithography used in semiconductor technology. Therein, lithography apparatus are used to define structures on a target, e.g. a silicon wafer. (Throughout this disclosure, the terms target and substrate are used interchangeably.) In order to define a desired pattern on a substrate wafer, the wafer is covered with a layer of a radiation sensitive photoresist. Afterwards, a desired structure is imaged onto the photoresist by means of a lithography apparatus, and the photoresist is then patterned by partial removal according to the pattern defined by the previous exposure step and then used as a mask for further structuring processes such as etching. In another important application the pattern may be generated by direct patterning without a resist, for example ion milling or reactive ion beam etching or deposition.
In 1997, I. L. Berry et al., in J. Vac. Sci. Technol. B, 15(6), 1997, pp. 2382-2386, presented a writing strategy based on a blanking aperture array and an ion projection system. Arai et al., U.S. Pat. No. 5,369,282, discuss an electron-beam exposure system using a so called blanking aperture array (BAA) which plays the role of a pattern definition means. The BAA carries a number of rows of apertures, and the images of the apertures are scanned over the surface of the substrate in a controlled continuous motion whose direction is perpendicular to the aperture rows. The rows are aligned with respect to each other in an interlacing manner to that the apertures form staggered lines as seen along the scanning direction. Thus, the staggered lines sweep continuous lines on the substrate surface without leaving gaps between them as they move relative to the substrate, thus covering the total area to be exposed on the substrate.
The above-mentioned article of Berry et al. describes a pattern definition device comprising a “programmable aperture array” with an array of 3000×3000 apertures of 5 μm side length with an n=4 alignment of rows and staggered lines. The article proposes to use a 200× demagnification ion-optical system for imaging the apertures of the BAA onto the substrate.
Starting from Berry's concept, E. Platzgummer et al., in the U.S. Pat. No. 6,768,125, presented a multi-beam direct write concept, dubbed PML2 (short for “Projection Maskless Lithography”), employing a pattern definition device comprising a number of plates stacked on top of the other, among them an aperture array means and a blanking means. These separate plates are mounted together at defined distances, for instance in a casing. The aperture array means has a plurality of apertures of identical shape defining the shape of beamlets permeating said apertures, wherein the apertures are arranged within a pattern definition field composed of a plurality of staggered lines of apertures, wherein the apertures are spaced apart within said lines by a first integer multiple of the width of an aperture and are offset between neighboring lines by a fraction of said integer multiple width. The blanking means has a plurality of blanking openings arranged in an arrangement corresponding to the apertures of the aperture array means, in particular having corresponding staggered lines of blanking openings. The teaching of the U.S. Pat. No. 6,768,125 with regard to the architecture and operation of the pattern definition device are hereby included as part of this disclosure by reference.
The main advantage of the PML2 multi-beam direct write concept is the large enhancement of the writing speed compared to single beam writers (multi-beam approach means a charged particle beam consisting of a plurality of sub-beams dynamically structured by an aperture plate including switch-able blanker devices). The improved productivity mainly arises from the following features:                The required current density is significantly reduced (relaxed source requirement);        The required single beam blanking rate can be limited to the low MHz-regime;        The importance of space charge is reduced (current is distributed to a large cross section when a broad beam is used);        Enhanced pixel transfer rate due to parallel writing strategy (instead of sequential raster scan);        High degree of redundancy possible using a plurality of beams (usable, for example, for gray scale generation).        
However, the PML2 has a number of challenges with respect to multi-beam patterning:                All beams need to have a generally identical dose rate (i.e. number of particles per gray level pixel exposure);        All beams need to have generally identical shape;        All beams need to be positioned on the target on a highly regular grid, which would require a practically distortion-free imaging and full control of current-dependent (=pattern-dependent) image distortion and de-focusing;        The need for a very precise wafer scanning system with sophisticated beam tracking (including correction of image placement, image rotation, image magnification and image distortion errors).        
The U.S. Pat. No. 7,276,714 of the applicant/assignee discloses a pattern definition means for particle beam processing, comprising at least an aperture plate and blanking means. The apertures in the aperture plate are arranged in “interlocking grids”, i.e., the apertures are arranged in groups in squares or rectangles whose basic grids are meshed together. This means that the positions of the apertures taken with respect to a direction perpendicular to a scanning direction and/or parallel to it are offset to each other by not only multiple integers of the effective width of an aperture taken along said direction, but also by multiple integers of an integer fraction of said effective width. In this context, “scanning direction” denotes the direction along which the image of the apertures formed by the charged-particle beam on a target surface is moved over the target surface during an exposure process.
This leads to a finer resolution on the target surface even though the individual spots formed by each image of an individual aperture are not decreased in size. Particular values of the fractional offsets are integer multiples of ½N times the effective width of an aperture, where N is a positive integer.
The state of the art PML2 concept is a strategy where the substrate is moved continuously, and the projected image of a structured beam generates 100 percent of the gray pixels by subsequent exposures of apertures located in line. To realize gray levels, the total amount of apertures in line is subdivided into columns, the number of columns corresponding to the number of desired gray levels. In a recent variant described in US Patent Application Publication No. US-2008/0237460 A1 by the applicant/assignee, a so called “trotting mode” writing strategy is proposed in which for each pixel one or a few beams along the (mechanical) scanning direction are used to generate the entire set of the gray pixels. The advantage of this variant is the reduced complexity of the CMOS structure and improved data management.
In all writing strategies based on a bitmap type of pattern coding, accurate placement control of the individual beams with respect to an ideal grid (physical grid, typically 20 nm pitch) is essential in order to fulfill lithographic requirements. While basically a systematic distortion of the image would cause a non-isotropic change of the blur, in the mentioned “trotting mode” a systematic distortion would also give rise to significant distortion of the pattern eventually generated as here only one beam (or a few, depending on the chosen strategy and redundancy) contribute to the total exposure dose of one pixel. Similarly, the dose per level, which is directly related to the aperture size and current density at the respective position on the aperture plate, becomes more important in the case of the trotting mode.
In the article “Overview of the Ion Projection Lithography European MEDEA and International Program” (SPIE Conference on Microlithography, Santa Clara, USA, Feb. 28-Mar. 1, 2000), R. Kaesmaier and H. Löschner discuss a correction of the pattern position in a stencil mask, introducing a X/Y placement correction such the ion-optical pattern transfer is achieved with virtually zero distortion at the wafer level.